Ethylene glycol is a valuable industrial compound that is widely employed as starting material for the manufacture of polyester fibres and polyethylene terephthalate (PET) resins. It also finds application in automotive antifreeze and hydraulic brake fluids, aircraft de-icers as well as in pharmaceutical products.
Ethylene glycol is typically prepared from ethylene oxide, which is in turn prepared by the silver-catalyzed oxidation of ethylene. More specifically, ethylene and oxygen are passed over a silver-based epoxidation catalyst, typically at pressures of 10-30 bar and temperatures of 200-300° C., producing a product stream comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water. In one well-known process, ethylene oxide is then reacted with a large excess of water in a non-catalytic process, producing a glycol product stream comprising close to 90 wt. % monoethylene glycol (MEG), the remainder being predominantly diethylene glycol (DEG), some triethylene glycol (TEG) and a small amount of higher homologues. In another well-known process, ethylene oxide is reacted with carbon dioxide in the presence of a catalyst to produce ethylene carbonate, which is subsequently hydrolyzed to provide ethylene glycol. Reaction via ethylene carbonate significantly improves the selectivity of ethylene oxide conversion to monoethylene glycol.
In the last few decades, many efforts have been directed towards the development of simplified processes and equipment for producing alkylene glycols from alkylenes, notably ethylene glycol from ethylene. For example, GB2107712 describes a process for preparing monoethylene glycol wherein the gases from the ethylene oxide reactor are supplied directly to a reactor wherein ethylene oxide is converted to ethylene carbonate or to a mixture of ethylene glycol and ethylene carbonate.
EP 0776890 describes a process wherein the gases from the ethylene oxide reactor are supplied to an absorber, wherein the absorbing solution mainly contains ethylene carbonate and ethylene glycol. The ethylene oxide in the absorbing solution is supplied to a carboxylation reactor and allowed to react with carbon dioxide in the presence of a carboxylation catalyst. The ethylene carbonate in the absorbing solution is subsequently supplied, with the addition of water, to a hydrolysis reactor and subjected to hydrolysis in the presence of a hydrolysis catalyst.
EP2178815 describes a reactive absorption process for preparing monoethylene glycol, wherein the gases from the ethylene oxide reactor are supplied to a reactive absorber and the ethylene oxide is contacted with an aqueous lean absorbent in the presence of one or more carboxylation and hydrolysis catalysts, and wherein the majority of the ethylene oxide is converted to ethylene carbonate or ethylene glycol in the absorber.
In each of these instances, considerable volumes of process water are produced. It is often desirable from an economic perspective to recycle as much of the process water as possible, for example, by recycling the process water to the ethylene oxide absorber or reactive absorber (e.g., for use as lean absorbent). Advantageously, such recycling not only reduces operating costs because it reduces the requisite amount of fresh water to be supplied to the process, but it may also reduce costs associated with disposal of the process water as waste. Further, re-use is desirable because the purging of considerable volumes of process water may also raise environmental concerns and/or disposal issues.
However, process water often contains various impurities, which are often a result of the formation of byproducts during the production of ethylene oxide, ethylene carbonate and/or ethylene glycol. For example, an overhead stream withdrawn from an ethylene oxide absorber or reactive absorber typically comprises, in addition to water, hydrocarbon impurities such as formaldehyde, acetaldehyde, etc. Additionally, organic chloride impurities may also be found in process water due to the use of organic chloride moderators in the epoxidation reaction.
If process water is recycled to the ethylene oxide absorber or reactive absorber without first removing at least a portion of these impurities, then over time, the impurities accumulate and are deleterious to the overall quality of the resulting glycol product and/or cause catalyst degradation.
U.S. Pat. No. 6,184,423 to Zo-Chun Jen discloses a process for removing acid impurities from process water in an ethylene glycol plant by utilizing anionic resins to adsorb formic acid and acetic acid from the process water. However, removal of impurities from a process water stream without the use of such resins would be advantageous from a cost standpoint. Furthermore, only acidic impurities may be removed using such resins.
Thus, the present inventors have sought to provide improved processes and systems for the production of ethylene carbonate and/or ethylene glycol. In particular, the present inventors have sought to provide processes and systems that reduce the amount of impurities in an aqueous process stream so that it may be recycled in the production of ethylene carbonate and/or ethylene glycol and have further sought to reduce the energy consumption associated with such recycling.